Oil cooler

Abstract

A transmission oil cooler has spaced elongated plates secured at the margins and ports at either end to define an oil flow channel. A center disposed between the plates for creating turbulence in the oil and enhancing heat transfer comprises a metal sheet folded to form generally planar fins in side-by-side relationship and the fins having louvers extending over most of their area. The center is disposed in the flow channel with the planes of the fins transverse to the oil flow or alternatively with the planes of the fins parallel to the oil flow.

Description

FIELD OF THE INVENTION

This invention relates to oil coolers and particularly to oil coolers having centers for high efficiency heat transfer.

BACKGROUND OF THE INVENTION

Transmission oil coolers for automotive vehicles are often installed in the vehicle radiator so that the engine coolant flows over the oil cooler and heat is transferred from the transmission oil to the engine coolant, The oil cooler should be characterized by compactness to fit within the tank of a radiator, low resistance to oil flow, strength to contain the pressure of the transmission oil, and high efficiency of heat transfer. The heat transfer efficiency and size are related since a smaller unit may be used for a given thermal transfer requirement if the efficiency is increased.

Heretofore, oil coolers have used a plate type heat exchanger comprising at least one pair of spaced plates secured together at their margins to define a passageway which contains the oil flow and has a conductive insert or center to enhance the heat transfer. Such a prior art device is shown in FIG. 1 and includes a female strip 10 with upstanding side margins 12 and a cooperating male strip 14 secured to the side margins 12 to form an elongated enclosure. A port 16 at each end of the assembly allow oil flow in one port, through the enclosure and out of the other port. A center 18, best shown in FIG. 2, comprises a stamped metal foil formed into staggered step-like undulations. The stamped foil is brazed to the plates or strips 10 and 14. The passageway between the plates is then configured by the center 18 into a plurality of meandering flow paths having a combined resultant flow in the direction shown by the arrow 19. The center 18 causes turbulence which enhances heat transfer and conducts heat from the oil to the plates 10, 14, thereby improving efficiency over a plain plate pair without a center 18.

Other types of heat exchangers have used other kinds of centers. U.S. Pat. No. 4,693,307 to Scarselletta shows a center design used in tube and fin heat exchangers suitable for automotive radiators. In that patent a radiator design is disclosed wherein a corrugated sheet formed into a plurality of side-by-side fins is sandwiched between flat tubes carrying engine coolant, with the fins being used to dissipate heat from the tubes to the air which flows through the fins. A conventional multi-louver fin is shown in FIG. 11 of Scarselletta which has louvers struck out of the plane of each fin and the louvers cover most of the fin area. Other fin designs shown in that patent are the hybrid fin variety which alternates plain fin surface with louvered areas.

The heat exchanger designs for radiators, do not directly apply to oil coolers because of differing constraints on size, corrosion resistance, pressure and thermal capacity. Thus different materials are used and the fin heights are different. To obtain the pressure capability and corrosion resistance for oil coolers, steel centers are used rather than aluminum which is commonly used for radiators and the center height is much less for the steel due to the lower heat conductivity of steel. Thus a number of design considerations require independent designs for radiators and oil coolers.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to further improve the thermal transfer efficiency of oil coolers.

The invention is carried out by a pair of spaced plates secured at their margin to form a flow passage, and a center between the plates having louvered fins. The invention also comprises aligning the center in the passage so that the planes of the fins are transverse to the flow direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings wherein like references refer to like parts and wherein:

FIG. 1 is a partly broken away view of a prior art oil cooler,

FIG. 2 is a detail view of a center for the cooler of FIG. 1,

FIG. 3 is a partly broken away isometric view of an oil cooler according to the invention,

FIG. 4 is a partly broken away enlarged segment of the oil cooler of FIG. 3,

FIG. 5 is a sectional view of a portion of the oil center of FIG. 3,

FIG. 6 is an assembly of plate pairs forming a larger cooler according to the invention,

FIG. 7 is a graph of heat transfer comparing the efficiency of the cooler according to the invention with the prior art cooler,

FIG. 8 is a partly broken away isometric view of an oil cooler according to another embodiment of the invention, and

FIG. 9 is a sectional view of a portion of the oil center of FIG. 8.

DESCRIPTION OF THE INVENTION

The heat exchanger described herein has been specifically developed as a transmission oil cooler for incorporation in the tank of an automotive radiator. Significant gains in efficiency have been experienced relative to the conventional oil cooler of FIGS. 1 and 2. The prior art oil center is limited by manufacturability to a low density of fins per inch, The new oil center described herein can have a density 2.5 times greater than the prior art center, thus enhancing the surface area and the heat transfer capacity.

Referring to FIG. 3, the oil cooler according to the invention comprises an elongated female plate 10 having upwardly turned margins 12 and a mating male plate 14 secured to the margin in fluid tight relationship and inlet and outlet ports 16 in either end of the plates as in the prior art configuration of FIG. 1. A center 20 is sandwiched between the plates 10, 14 and brazed to the plates to assure mechanical strength of the assembly and excellent thermal coupling of the center and the plates. The center 20, as best shown in FIG. 4, comprises a corrugated or folded sheet of foil forming generally planar fins 22 in side-by-side relationship and joined by bends 24. Each of the fins 22 has a set of louvers 26 extending over most of the fin area. The fins 22 extend transverse to the direction of oil flow as indicated by the arrow 28 in FIG. 3. All the oil must then flow through the louvers 26 of each fin to pass from the inlet to the outlet.

FIG. 5 illustrates a pair of neighboring fins in cross section and the oil flow shown by flow lines 30 passing through the louvers. The louvers 26 must then be sufficiently open to permit flow without undue restriction. To accommodate free flow but still creating turbulence, the louvers are turned from the plane of the fin by an angle on the order of 70 .degree.. Preferably the louvers are arranged in groups with several louvers, say, 4 to 10, per group with neighboring groups angled in opposite directions. A specific structure according to the invention used mild steel center material about 0.05 to 0.15 mm thick formed into fins having a pitch p (FIG. 4) of about 1.7 mm and having a peak to peak height of about 3.4 mm. The louvers each have a width w (FIG. 5) of about 1.14 mm.

The single plate pair of FIG. 3 may be used as a cooler or several plate pairs may be stacked up and joined at their ports 16 to form a cooler as shown in FIG. 6. Such a cooler using four plate pairs yielded the heat transfer results shown in FIG. 7. The upper curve A shows the heat transfer at different oil flow rates for the FIG. 3 design while curve B is the corresponding results for the prior art oil center of FIGS. 1 and 2. The vast improvement enables fewer plates to be used in a cooler to obtain comparable cooling or to use the same size cooler to obtain better cooling. In vehicle testing, a transmission oil cooler according to the invention operated at vehicle speeds of 50 mph resulted in a transmission sump temperature 18 .degree. F. cooler than the prior art cooler under the same conditions. This transverse center design is particularly adapted to use at low flow rates since it causes turbulence in the flow to aid in heat transfer. Thus coolers with flow rates yielding low Reynolds numbers and tending to give laminar flow benefit from the transverse center design.

Another embodiment of the invention is shown in FIG. 8 which is similar to FIG. 3 except that the center 20' is oriented with the fins 22' extending parallel to the general direction of oil flow. That is, the center comprises a sheet folded to form generally planar fins in side-by-side relationship and joined at bends like that shown in FIG. 4 and the fin size and pitch and louver size is the same as in the FIGS. 3-5 embodiment. The oil flow generally parallel to the plane of the fins is displayed by flow lines 30' in FIG. 9 which is a cross section of the fins 22'. Thus the primary flow is not through the louvers but some oil does pass through the louvers 26' to cause turbulence and prevent the occurrence of a boundary layer along the fins. Since the oil flow is different from the FIG. 3 embodiment, the louver angle is selected to optimize the heat transfer and oil flow characteristics. With the flow parallel to the plane of the fins, the preferred angle is on the order of 30*. This structure has heat transfer efficiency comparable to the FIG. 3 embodiment. This parallel center design is advantageous at high flow rates which yield high Reynolds numbers. At such high flow rates the flow is turbulent and little heat transfer advantage would be gained by using the transverse centers which create more turbulence and which also have a higher pressure drop than the parallel centers.

Claims

1. An oil cooler for transferring heat from oil within the cooler to fluid without the cooler, comprising:

a pair of elongated plates secured together at their margins and spaced from one another between the margins to form an oil flow path, the plates having inlet and outlet ports to define the general direction of oil flow,

an oil center between the plates in thermal contact with the plates and in the oil flow path for transferring heat from the oil to the plates,

the oil center comprising a corrugated metal sheet having a plurality of substantially plane fins in side-by-side relationship and joined at bends wherein the bends make the thermal contact with the plates, and

a plurality of louvers in each fin for creating turbulence in the oil flow and defining openings in the fins, wherein the oil center is disposed with the direction of the fin planes transverse to the general direction of flow so that the oil passes through the openings of the fins.

2. The invention as defined in claim 1 wherein the oil center is disposed with the direction of the fin planes parallel to the general direction of flow.

3. The invention as defined in claim 1 wherein the oil center is disposed with the direction of the fin planes parallel to the general direction of flow and the louvers project from the fin planes at an angle on the order of 30.degree..

4. The invention as defined in claim 1 wherein the louvers project from the fin planes at an angle on the order of 70.degree..